The present invention relates to a lead frame used to make a semiconductor package. More particularly, the present invention relates to a lead frame capable of a superior resin filling profile in a molding process used to make the semiconductor package.
Typically, lead frames for semiconductor packages are fabricated by processing a strip, made of copper (Cu), iron (Fe), aluminum (A1) or an alloy thereof, in accordance with a mechanical method such as a stamping method or a chemical method such as an etching method in such a fashion that it has a plurality of leads. Leads of such a lead frame serve as conductive lines for connecting a semiconductor chip mounted on the lead frame to external circuits. Such leads also serve as a support for holding a semiconductor package fabricated using the associated lead frame to a mother board. Lead frames formed on one strip are cut at their peripheral edges in a singulation process, so that they are separated from one another.
Referring to FIG. 1a, a typical structure of a conventional lead frame is illustrated. As shown in FIG. 1a, the lead frame, which is denoted by the reference numeral 10xe2x80x2, has a structure including a central opening 14xe2x80x2 having a substantially rectangular or square shape, and a plurality of leads extending radially around the central opening 14xe2x80x2. Each of the leads has an inner lead 11xe2x80x2 adapted to be encapsulated by a resin encapsulate (denoted by the reference numeral 4 in FIG. 3) subsequently molded, and an outer lead 12xe2x80x2 disposed beyond the resin encapsulate. Each lead is connected to a dam bar 19xe2x80x2 at the outer end of its inner lead 11xe2x80x2 and at the inner end of its outer lead 12xe2x80x2 so that it is supported by the dam bar 19xe2x80x2. In addition to the support function for the leads, the dam bar 19xe2x80x2 has a function for preventing melted encapsulating resin from being outwardly leaked between adjacent inner leads 11xe2x80x2 during a molding process.
Adjacent ones of the inner leads 11xe2x80x2 of the lead frame have a space defined therebetween in such a fashion that it increases gradually in width as it extends from the opening 14xe2x80x2 to the dam bar 19xe2x80x2. Also, all spaces defined for all inner leads 11xe2x80x2 have the same width at the same radial position. In other words, all spaces of all inner leads 11xe2x80x2 have the same size and shape.
Pseudo tie bars 15xe2x80x2 are arranged at four corners of the lead frame 10xe2x80x2, respectively. Each pseudo tie bar 15xe2x80x2 extends diagonally while having a width larger than a typical width of the leads. Where a semiconductor chip mounting plate (not shown) is to be arranged within the central opening 14xe2x80x2, the pseudo tie bars 15xe2x80x2 serve as tie bars for supporting the semiconductor chip mounting plate by use of an adhesion means such as an adhesive tape. Otherwise, the pseudo tie bars 15xe2x80x2 may be removed to simply leave spaces, respectively. In some cases, they may be used as inner leads or ground leads.
In FIG. 1, the reference numeral 18xe2x80x2 denotes an initial encapsulating resin introduction region defined at a selected one of the pseudo tie bars 15xe2x80x2 respectively arranged at the four corners of the lead frame 10xe2x80x2 in order to allow melted encapsulating resin of high temperature and high pressure to be introduced into a molding region.
As apparent from the above description, the conventional lead frame 10xe2x80x2 has a symmetrical structure in longitudinal, lateral, and diagonal directions.
On the other hand, FIG. 1b is a schematic view illustrating a procedure for fabricating a conventional heat sink. As shown in FIG. 1b, a pair of facing U-shaped slots 51xe2x80x2 are formed through a metal strip 50xe2x80x2 in accordance with a stamping process in such a fashion that a pair of facing support bars 52xe2x80x2 are left therebetween. The support bars 52xe2x80x2 have a reduced thickness as compared to that of the metal strip 50xe2x80x2 because the metal material of the metal strip 50xe2x80x2 is subjected to an elongation at regions corresponding to those support bars 52xe2x80x2 during the stamping process. The reason why the support bars 52xe2x80x2 are formed is because when a heat sink, which is denoted by the reference numeral 5xe2x80x2, is completely cut from the metal strip 50xe2x80x2 using a single stamping step, there is a high possibility for the heat sink 5xe2x80x2 to be bent due to a relatively large thickness (typically, about 1.65 mm) of the metal strip 50xe2x80x2. In order to planarize bent heat sinks, it is necessary to use an additional process. After the stamping process, the heat sink 5xe2x80x2 is still held to the metal strip 50xe2x80x2 while being supported by the support bars 52xe2x80x2 between the slots 51xe2x80x2. In this state, the support bars 52xe2x80x2 are cut, so that the heat sink 5xe2x80x2 is separated from the metal strip 50xe2x80x2. The resultant heat sink 5xe2x80x2 has a substantially square structure provided with two protrusions 5bxe2x80x2. The support bars 52xe2x80x2 are cut in accordance with upward and downward pressing operations respectively conducted by two pressing tools. Since the support bars 52xe2x80x2 are relatively thick, the elongation of the metal thereof occurring during the pressing operations proceeds in directions slightly inclined along an associated one of the support bars 52xe2x80x2 from upward and downward directions, respectively. As a result, each protrusion 5bxe2x80x2 has a V-shaped cross-section at each side wall thereof.
Typically, the cut heat sink 5xe2x80x2 is subsequently coated with nickel (Ni) in order to prevent its surface, exposed in a state integrated into a resin encapsulate, from being oxidized in air. The nickel-coated surface of the heat sink 5xe2x80x2 is also subjected to a sand blast process for an easy marking thereof. The heat sink 5xe2x80x2 is also subjected to a well-known black oxidation process (adapted to form a CuO thin film and/or a Cu2O thin film) at its surface, on which a semiconductor chip is mounted, and its surface contacting the resin encapsulate, in order to obtain an improved bonding force to the resin encapsulate at those surfaces.
FIG. 2 is a cross-sectional view illustrating a typical mold used in a molding process for the fabrication of semiconductor packages. As shown in FIG. 2, the mold, which is denoted by the reference numeral 20, includes an upper mold 21 and a lower mold 22. Typically, the lead frame 10xe2x80x2 attached at its lower surface with the heat sink 5xe2x80x2 and at its upper surface with a semiconductor chip 1xe2x80x2 is laid on the lower mold 22 which is, in turn, coupled to the upper mold 21 in such a fashion that the lead frame 10xe2x80x2 is received in a mold cavity 23 defined by the upper and lower molds 21 and 22. A pressurized melted encapsulating resin 4xe2x80x2 is injected into the mold cavity 23 via a mold runner 24 by a resin feeding ram 26 arranged at a pouring gate 25 of the mold 20. The mold runner 24 is formed on the lower surface of the upper mold 21 in such a fashion that it communicates with the mold cavity 23 while communicating with a port 27. The melted encapsulating resin of high temperature and high pressure is set as it is cooled, thereby forming a resin encapsulate (denoted by the reference numeral 4 in FIG. 3).
FIG. 3 is a cross-sectional view illustrating a typical structure of a conventional semiconductor package fabricated using a lead frame such as the conventional lead frame 10xe2x80x2 of FIG. 1a and a heat sink such as the conventional heat sink 5xe2x80x2 of FIG. 1b. In FIG. 3, elements respectively corresponding to those in FIGS. 1a and 1b are denoted by the same reference numerals. As shown in FIG. 3, the semiconductor package, which is denoted by the reference numeral 1xe2x80x2, includes a heat sink 5xe2x80x2 having a relatively large thickness, and a plurality of leads each having an inner lead 11xe2x80x2 and an outer lead 12xe2x80x2. The leads are attached to the outer peripheral portion of the upper surface of the heat sink 5xe2x80x2 by means of adhesive tapes 6a. A semiconductor chip 2 integrated with a variety of circuits is centrally mounted on the heat sink 5xe2x80x2 by means of an adhesive layer 6. The semiconductor package 1xe2x80x2 also includes a plurality of conductive wires 3 each connecting each inner lead 11xe2x80x2 to the semiconductor chip 2, and a resin encapsulate 4 for protecting the semiconductor chip 2, inner leads 11xe2x80x2 and conductive wires 3 from the external environment. The heat sink 5xe2x80x2 is exposed at its surface arranged at the lower surface side of the resin encapsulate 4 in order to obtain improved heat discharge characteristics. The heat sink 5xe2x80x2 is provided with two protrusions 5bxe2x80x2 formed in a pressing process for the heat sink 5xe2x80x2, as mentioned above.
In the case of a semiconductor package fabricated using a conventional lead frame having both a semiconductor chip mounting plate and a heat sink, although not shown, a semiconductor chip is mounted on the semiconductor chip mounting plate by means of an adhesive layer. In the fabrication of such a semiconductor package, the lead frame attached with the semiconductor chip is loaded in a mold in a state in which the heat sink has been loaded in the cavity of a lower mold included in the mold. Subsequently, a melted encapsulating resin of high temperature and high pressure is injected into the cavity of the mold, and then set. That is, the lead frame structure and molding method used in the fabrication of this semiconductor package are the same as the lead frame structure using no semiconductor chip mounting plate and the molding method associated therewith, except that the heat sink is attached to the lower surface of the semiconductor chip mounting plate using the pressure and bonding force of the encapsulating resin.
In the above mentioned conventional lead frame 10xe2x80x2, however, the space defined between adjacent ones of the inner leads 11xe2x80x2 arranged at the initial encapsulating resin introduction region 18xe2x80x2 is the same as that of the inner leads 11xe2x80x2 arranged at regions other than the initial encapsulating resin introduction region 18xe2x80x2. For this reason, the filling profile of the melted encapsulating resin injected into the mold cavity through the initial encapsulating resin introduction region 18xe2x80x2 is degraded. That is, although the initial encapsulating resin introduction region 18xe2x80x2 defined at one corner of the lead frame 10xe2x80x2 is subjected to a higher pressure than those applied to regions, namely, air vent regions (denoted by no reference numeral), defined at the remaining three corners of the lead frame 10xe2x80x2, the space defined between adjacent inner leads 11xe2x80x2 is uniform at all regions, so that it is impossible to obtain a smooth flow of melted encapsulating resin. As a result, a turbulent flow of melted encapsulating resin is generated, thereby causing voids to be formed in the molded resin encapsulate 4 and/or at the interface between the molded resin encapsulate 4 and mold cavity 23. Consequently, there is a high possibility of a degradation in the performance characteristics and appearance of the final product.
It is also difficult to cut the conventional heat sink 5xe2x80x2 of FIG. 1b used in the fabrication of the conventional semiconductor package 1xe2x80x2 by use of a single stamping step because the heat sink 5xe2x80x2 has a large thickness of 1 to 3 mm. For this reason, several stamping steps are required for the cutting of the heat sink 5xe2x80x2. This results in a complexity of the entire process. The entire process becomes more complex because the heat sink 5xe2x80x2 is coated with nickel (Ni) in order to prevent its surface, exposed in a state integrated to a resin encapsulate, from being oxidized in air, and then subjected to a sand blast process for an easy marking thereof while being subjected to a well-known black oxidation process at its surface, on which a semiconductor chip is mounted, and its surface contacting the resin encapsulate, in order to obtain an improved bonding force to the resin encapsulate at those surfaces. In addition, the protrusions 5b left after the cutting of the heat sink 5xe2x80x2 serve to render the turbulent flow generated in a resin filling process to be severe, thereby increasing the possibility of the formation of voids. As a result, the quality of the molded resin encapsulate 4 is degraded.
In the fabrication of the conventional semiconductor package 1xe2x80x2 using the conventional lead frame 10xe2x80x2 of FIG. 1a and the conventional heat sink 5xe2x80x2 of FIG. 1b, additional problems are also involved. For example, the leads of the lead frame 10xe2x80x2 may be downwardly bent due to the weight of the heat sink 5xe2x80x2 during the feeding of the lead frame 10xe2x80x2 to a subsequent process in a state in which each inner lead 11xe2x80x2 is bonded to the peripheral portion of the upper surface of the heat sink 5xe2x80x2. This is because the heat sink 5xe2x80x2 has a thickness considerably larger than that of each lead 11xe2x80x2. Due to such a deformation of the leads 11xe2x80x2, a short circuit may be generated between adjacent ones of the leads 11xe2x80x2. Also, the quality of the wire bonding formed between the semiconductor chip 1 and each lead 11xe2x80x2 may be degraded.
In the conventional semiconductor package structure, the heat sink 5xe2x80x2 is flush with the lower surface of the resin encapsulate 4 at its lower surface in order to obtain an improved heat discharge effect. For such a structure, it is necessary to use a more sophisticated mold structure. That is, it is necessary to use a mold capable of allowing the heat sink 5xe2x80x2 to be accurately flush with the lower surface of the cavity of the lower mold 22 at its lower surface. Moreover, it is impossible to completely eliminate a flashing phenomenon of the melted encapsulating resin between the lower surface of the heat sink 5xe2x80x2 and the lower surface of the cavity of the lower mold 22 because the injection of the melted encapsulating resin is conducted at a high temperature and a high pressure. For this reason, it is necessary to use a deflashing process for removing a set resin thin film left on the exposed lower surface of the heat sink 5xe2x80x2.
Therefore, a primary object of the invention is to provide a lead frame capable of exhibiting a superior resin filling profile, thereby avoiding formation of voids in a molding process.
A secondary object of the invention is to provide a semiconductor package fabricated using a heat sink, which is adapted to a lead frame structured to accomplish the primary object of the invention and has a structure capable of achieving an easy fabrication thereof, effectively suppressing a bending phenomenon of leads of the lead frame, and minimizing generation of a turbulent flow of melted encapsulating resin in a molding process.
In accordance with one aspect, the present invention provides a lead frame comprising: a plurality of spaced inner leads extending outwardly in a radial direction around an opened central chip mounting region, the inner leads having outer ends integrally supported by a dam bar having four corners, inside the dam bar, respectively; a plurality of outer leads each having an inner end supported by the dam bar outside the dam bar and an outer end supported by a frame; pseudo tie bars extending diagonally from three of the four corners of the dam bar toward the central chip mounting region, respectively; an initial encapsulating resin introduction region defined by at least two of the inner leads arranged adjacent to the remaining one of the four corners of the dam bar, where any one of the pseudo tie bars does not exist, the initial encapsulating region having an encapsulating resin introduction slot having a width larger than that of a space defined between adjacent ones of the remaining inner leads; air vent regions respectively defined at the three corners of the dam bar where the pseudo tie bars are arranged, each of the air vent regions including an associated one of the pseudo tie bars, and at least two of the inner leads arranged adjacent to the associated pseudo tie bars; and at least the inner leads arranged at the initial encapsulating resin introduction region having a width increasing gradually as they extend from a region adjacent to the dam bar to a region adjacent to the central chip mounting region in such a fashion that a space defined between adjacent ones of the inner leads arranged at the initial encapsulating resin introduction region has a gradually reduced width.
In accordance with another aspect, the present invention provides a semiconductor package comprising: semiconductor chip; a heat sink having an upper surface, on which the semiconductor chip is centrally mounted by an adhesive layer interposed between the semiconductor chip and the heat sink; a lead frame having a plurality of radially-extending leads each being bonded to a peripheral edge of the heat sink by the adhesive layer and electrically connected to the semiconductor chip by a wire, the lead frame also having an encapsulating resin introduction region and air vent regions, each of the regions overlapping with a part of the leads; a resin encapsulate completely encapsulating the semiconductor chip, the wires, and the heat sink while encapsulating the initial encapsulating resin introduction region and the air vent regions, and the inner leads; and at least two of the inner leads arranged at the initial encapsulating resin introduction region having a width increasing gradually as they extend from a region adjacent to a peripheral edge of the resin encapsulate to a region adjacent to the semiconductor chip in such a fashion that a space defined therebetween has a width gradually reduced, but larger than that of a space defined between adjacent ones of the inner leads arranged regions other than the initial encapsulating resin introduction region.